C++ Insights

Source:

#include <iostream> // std::cout

#include <type_traits>

template <class T> // deductions among universal reference
void fun(T &&b)    // fold reference, maintain const
{
    std::cout << b << " ";
    std::cout << std::is_const<T>::value << " ";
    std::cout << std::is_lvalue_reference<T>::value << " ";
    std::cout << std::is_rvalue_reference<T>::value << std::endl;
}

int main()
{
    int a = 1;
    fun(a);            // lvalue: T=int&
    fun(std::move(a)); // rvalue: T=int

const int ca = 1;
    fun(ca);            // const_lvalue: T=int& （why no const?）
    fun(std::move(ca)); // const_rvalue: T=const int

int &la = a;
    fun(la); // lvalue_ref: T=int&

int &&ra = 1;
    fun(ra);                             // rvalue_ref: T=int& (r_ref is a lval)
    fun(std::forward<decltype(ra)>(ra)); // rvalue_ref + perfect forwarding T=int

const int &cla = a;
    fun(cla); // const_lvalue_ref: T=int& (no const?)

const int &&cra = 1;
    fun(cra); // const_rvalue_ref: T=int& (no const?)

return 0;
}

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